Pivory.com: From Ajax to WebSocket with PHP, a Quick Example
WebSocket is the modern way for realtime two-way communications between server and browser. In the beginning we have Ajax calls [...] and with WS in picture the logic becomes [different]. [There] is a separate WS server, say listening on xxx.xxx.xxx.xxx:8080, and it can send messages to connected clients in realtime at any moment. The WS server can be in any language (such as node.js + socket.io) and on any machine or even in a cloud service. I use PHP (try http://socketo.me for Ratchet) because I am lazy.